The present invention relates to a novel photopolymerizable composition highly sensitive to actinic light rays, which is suitably used as a material for preparing a resin letterpress plate, a lithographic printing plate, a resist material such as a dry film resist or a resist for gravure printing, a photomask and a proof material.
The present invention also relates to a presensitized plate suitable for use in making a lithographic printing plate which allows printing without using any dampening water.
Photopolymerizable compositions have widely been used in various applications, in an industrial scale, for performing printing, copying and forming resists.
The composition in general comprises an ethylenically unsaturated compound or another type of polymerizable compound and a photopolymerization initiator or system and optionally comprises, for instance, a polymeric compound having film-forming ability, a heat polymerization inhibitor or a plasticizer. In addition, there has been well known a method for forming an image which comprises forming a solution of such a photopolymerizable composition, applying the solution to a substrate to form a light-sensitive material provided with a layer of the photopolymerizable composition (light-sensitive layer), imagewise exposing the light-sensitive layer through a desired original to polymerize and harden the photopolymerizable composition on the exposed area, and dissolving and removing the unexposed area by treating it with an organic solvent or an alkaline water capable of dissolving only the unexposed area to form an image composed of the hardened portion. In such a method, there have been used, for instance, benzyl, benzoin, anthraquinone, Michler's ketone and thioxanthone as photopolymerization initiators; and, for instance, oligoester (or polyester) unsaturated carboxylates obtained by ester condensation of esters of unsaturated carboxylic acids and aliphatic polyols, or oligoesters (or polyesters) of polyvalent carboxylic acids and aliphatic polyols with unsaturated carboxylic acids as ethylenically unsaturated compounds.
However, these photopolymerizable compositions have low sensitivity and thus it takes a long time to harden the compositions through imagewise exposure and hence they have low workability.
On the other hand, there have been proposed various kinds of presensitized plates (hereinafter referred to as "dry PS plates") for use in making a lithographic printing plate which allows printing without using any dampening water (hereinafter referred to as "dry lithographic printing plate"). Among these, dry PS plates which comprise a substrate provided thereon with, in order, a light-sensitive resin layer and a silicone rubber layer have particularly excellent properties and examples thereof include those disclosed in Japanese Patent Publication for Opposition Purpose (hereinafter referred to as "J. P. KOKOKU") Nos. Sho 54-26923 and Sho 55-22781.
The silicone rubber layer used in the dry PS plate is usually a polymer comprising, as a main skeleton, a polysiloxane chain, which is partially crosslinked with a crosslinking agent. As methods for hardening the silicone rubber layer, there have usually been known the following two methods:
(1) Condensed Type: a method in which an organopolysiloxane having hydroxyl groups on both ends is crosslinked with a silane or siloxane having a hydrolyzable functional group directly bonded to the silicon atom to give a silicone rubber; and PA1 (2) Addition Type: a method in which a polysiloxane having .tbd.Si--H groups is subjected to an addition reaction with a polysiloxane having -CH=CH- groups to give a silicone rubber (see, for instance, Japanese Unexamined Patent Publication (hereinafter referred to as "J. P. KOKAI") NO. Sho 61-73156 and Japanese Patent Appln. Serial No. (hereinafter referred to as "J. P. A. No.") Hei 1-301568 (=J. P. KOKAI No. Hei 3-161753)). PA1 (a) at least one compound obtained by reacting one mole of a polyamine represented by the following general formula (I) with at least 2 moles of a compound having a group capable of reacting with the polyamine and a polymerizable ethylenically unsaturated group: ##STR2## wherein n is an integer of not less than 1, and (b) a photopolymerization initiator. PA1 (a) at least one compound obtained by reacting one mole of a polyamine represented by the formula (I) with at least 2 moles of a compound having a group capable of reacting with the polyamine and a polymerizable ethylenically unsaturated group, PA1 (b) a photopolymerization initiator, and PA1 (c) a polymeric compound capable of forming a film.
The hardening properties of the condensed type silicone rubber layer and the adhesion thereof to a light-sensitive layer are influenced by the moisture content of the atmosphere during hardening. Therefore, if it is used in a dry PS plate, the sensitivity of the resulting dry PS plate is liable to cause scattering and thus it is difficult to produce dry PS plates having stable sensitivity. For this reason, the addition type silicone rubber is considered to be superior to the condensed type one since the former does not have such a drawback.
As the light-sensitive resin layer for positive-working PS plates, there have been used photopolymerizable light-sensitive compositions which are hardened through exposure to light. A method for forming images on a PS plate having such a layer structure generally comprises exposing the light-sensitive layer to light to harden the same or optionally photoadhering the light-sensitive layer to the uppermost silicone rubber layer at the interface therebetween to thus give non-image area comprising the silicone rubber layer. In this respect, these layers are firmly linked together and the penetration of a developer into the light-sensitive layer and the corresponding dissolution thereof are prevented. On the other hand, image areas are formed by permeating a developer into the unhardened light-sensitive resin layer through the silicone rubber layer to partially or completely dissolve the unhardened light-sensitive resin layer and then removing the silicone rubber layer on the unhardened light-sensitive layer through the application of a physical force. Images are thus formed on a dry PS plate. However, the dry PS plate has sensitivity lower than that of the usual PS plate for use in making a lithographic printing plate which requires the use of dampening water during printing and, therefore, a long imagewise exposure time is required during plate-making, which in turn leads to low workability.